Underwater masks are designed to provide scuba divers with a clear view of their surroundings.
Of paramount importance is the field of vision afforded by the mask. In fact, the diver is not only to see in front of him but also to gain an all-around view both in the sideward directions and in the pectoral direction, where a number of gauges and controls, such as the cummerbound of the Buoyancy Compensating Device (BCD), the weight belt, inflation system, etc., would usually be found, especially when scuba diving equipment is used.
Underwater masks currently in use have two front plates made of a transparent material, such as a glass or a plastics material, or alternatively a single full-face transparent plate or pane.
Such masks do provide front vision. The width of the viewfield may be increased by expanding the areas of the transparent plates and bringing them closer to the eye, but there remain restrictions to the peripheral vision.
There are several proposals for improving the peripheral vision.
French Patent No. 893,643 discloses a mask having a single pane for front vision which is positioned perpendicularly to the user's sight axis. In addition, the pane holder, being basically tubular in shape, is made of a transparent material.
This solution only expands the diver's viewfield to a limited extent because it takes into account neither the optical properties of the holder nor the lay of the holder walls relative to the eyes.
Additionally, the use of a single front transparent pane makes it difficult to impart it with special optical properties, such as a positive optical diopters for the shortsighted or a negative one for the longsighted.
U.S. Pat. No. 3,671,976 (Johnson et al.) discloses a mask having a single transparent pane mounted frontally and conforming with the face contour at the mask sides, said pane being located as close as possible to the eyes.
That invention provides a possible solution to the problem of obtaining adequate side vision, but fails to ensure good peripheral vision in the pectoral direction.